1.Field of the Invention
The present invention relates to a magnetooptical disk unit recording and reproducing signals in and from a magnetooptical recording medium, a signal recording method and a signal reproducing method.
2. Description of the Prior Art
A magnetooptical recording medium is watched with interest as a reloadable recording medium having a large storage capacity and high reliability, and now being put into practice as a computer memory or the like. Further, a magnetooptical recording medium having a storage capacity of 6.0 Gbytes is standardized under AS-MO (advanced storaged magnetooptical disk) standards, to be put into practice.
In addition, a magnetooptical recording medium having a recording capacity of 14 Gbytes is proposed according to an extensional magnetic domain reproducing system for extensionally transferring magnetic domains from a recording layer to a reproducing layer.
Referring to FIGS. 14A to 14C, a signal "01011001110" is recorded in a magnetooptical recording medium according to such an extensional magnetic domain reproducing system by applying an external magnetic field to the magnetooptical recording medium thereby forming domains 500 by "0", a domain 501 by "1", a domain 502 by "11", a domain 503 by "00" and a domain 504 by "111" respectively.
The signal is reproduced by irradiating the magnetooptical recording medium with a laser beam and applying an alternating magnetic field 505 of .+-.300 Oe thereby extensionally transferring each magnetic domain of a recording layer formed as a domain to a reproducing layer. In this case, the alternating magnetic field 505 is applied to central portions of respective domains having different domain lengths, such as the domain 501 having the minimum domain length and the domain 502 having a domain length 2T, for example.
In the conventional reproducing method, however, a leakage magnetic field 506 from the domain 501 having the minimum domain length and a leakage magnetic field 507 from the domain 502 having the domain length 2T are different from each other and hence the domains 501 and 502 are irradiated with the laser beam at different intensity levels. In order to reproduce the signal, therefore, the intensity of the laser beam must be varied with the domain lengths.
The intensity of the leakage magnetic field 506 from the domain 501 having the minimum domain length is increased from both ends toward the central portion of the domain 501, while the intensity of the leakage magnetic field 507 from the domain 502 having the domain length 2T is increased on both ends of the domain 502 and reduced on the central portion. Thus, the domain 501 is readily extensionally transferred from the recording layer to the reproducing layer when the alternating magnetic field 505 is applied to the central portion of the domain 501. However, it is difficult to extensionally transfer the domain 502 from the recording layer to the reproducing layer due to the small intensity of the leakage magnetic field 507 on the central portion. In order to extensionally transfer the domain 502 to the reproducing layer, therefore, the intensity of the laser beam applied to the domain 502 must be increased for increasing the intensity of the leakage magnetic field 507 on the central portion of the domain 502. Consequently, the intensity of the applied laser beam must be increased in proportion to the domain length. Some domain may not be extensionally transferred to the reproducing layer even if the intensity of the applied laser beam is increased, depending on the domain length.